The pilot-in-command (PIC) did not review the available graphical area forecast weather information and was not sufficiently informed to avoid the forecast icing conditions. The severe in-flight icing conditions caused an ice accumulation that the aircraft's ice-protection systems were unable to prevent or remove. As a result, the aircraft entered a power-on stall condition and an uncontrollable descent. The PIC did not detect the severe ice accumulation in sufficient time to alter the flight route to avoid the icing conditions.Findings as to Causes and Contributing Factors The pilot-in-command (PIC) did not review the available graphical area forecast weather information and was not sufficiently informed to avoid the forecast icing conditions. The severe in-flight icing conditions caused an ice accumulation that the aircraft's ice-protection systems were unable to prevent or remove. As a result, the aircraft entered a power-on stall condition and an uncontrollable descent. The PIC did not detect the severe ice accumulation in sufficient time to alter the flight route to avoid the icing conditions. Following an internal investigation into the occurrence, the company, as an interim safety action, distributed a memorandum to advise flight crews to review all available weather data before flights. The company has since developed a syllabus, examination and emergency checklist regarding severe icing and has implemented them as part of its training program to provide flight crews with more in-depth knowledge of severe icing conditions and exit strategies.Safety Action Taken Following an internal investigation into the occurrence, the company, as an interim safety action, distributed a memorandum to advise flight crews to review all available weather data before flights. The company has since developed a syllabus, examination and emergency checklist regarding severe icing and has implemented them as part of its training program to provide flight crews with more in-depth knowledge of severe icing conditions and exit strategies.